Abnormal brain maturation is believed to contribute to the path physiology and etiology of a number of mental health and psychiatric disorders, amongst them autism, schizophrenia, attention deficit and obsessive-compulsive disorder. A critical component of brain maturation is the process of myelination, the development of the fatty myelin layer surrounding white matter axons, which speeds information transfer between discrete brain regions. Despite the crucial role myelination plays in normal brain function, though the establishment and maintenance of these efficient communication pathways;to date little is known quantitatively about the spatial and temporal evolution of myelination. Moreover, the relationships between myelin maturation and cognitive and behavioral development, for example the evolution of motor coordination, language or visual reception, remain poorly understood. This proposal aims to address both of these deficiencies in knowledge, performing the first quantitative and longitudinal study of myelination during healthy neurodevelopment. Paired with synchronized cognitive and behavioral assessments, this data will provide a new vista of normal brain development. This normative dataset will be uniquely positioned to facilitate specific quantitative comparisons between healthy and suspected abnormal development, allowing researchers to identify and establish the spatial and temporal patterns of relevant deficits. The ultimate goals of this research are to quantitatively map the myelination trajectory over the first 5 years of life in a healthy population, and to examine relationships between myelin maturation and cognitive and behavioral development. To achieve this aim, a new method for quantitative myelin imaging, termed mcDESPOT, will first be further develop and optimized for use in pediatric participants. Using this optimized technique, myelin maturation trajectories throughout the brain will be reconstructed by acquiring high-spatial resolution, whole-brain myelin maps at incremented time points throughout development. Following 128 infants (3-24 months of age) and 128 toddlers (2-5 years), scanning and age-appropriate psychometric testing will be performed at effective 3 and 6- month intervals, respectively. Appropriately aligned, this data will result in the creation of 14 age-specific averaged myelin maps, providing the first quantitative description of myelin maturation in vivo. Relationships between these myelin measures and cognition will be investigated through comparison with the cognitive assessments of motor coordination, language and visual reception, providing new insight into how the brain normally develops and which brain regions are involved at each stage.